Mineable oil sands ore comprise bitumen, water, sand, fine clays, and silt. The bitumen may be separated from the remaining components of the oil sands using a hot water extraction process. In the hot water process, ore is mixed with hot water, and the oil floats to the top of the mixture. The water, sand, and silt are present below the oil. A caustic solution may be added to facilitate separation of bitumen from the sand. The oil is removed from the top of the mixture. The water, including clays and other fines from the ore which remain suspended in the water, is removed from the bottom of the mixture along with some remaining bitumen, and transported to settling ponds. Horizontal separators may have application in separating water from clay and other fines. An example of a horizontal separator is found in U.S. Pat. No. 2,622,794, issued Dec. 23, 1952 to Smith.
Fines in suspension settle to about 30 to 40% solid (w/w), at which point they form a gel-like material (a “colloidal suspension”; see below). The colloidal suspension includes large amounts of water and slows further settling from the water. In terms of weight, the smallest fines may represent about 3% of the mature fine tailings (“MFT”), but may entrap coarser solids such that the colloidal suspension forming from MFT may contain 30% by weight solids.
Water in the MFT cannot immediately be used again in the hot water extraction process, requiring that additional water be introduced into the system to continue the hot water process. MFT eventually settle in the ponds producing water that can be reused, but the residence time in the ponds can be years, requiring very large settling ponds which present a hazard to migrating water fowl and are a potential source of groundwater contamination (Mercier et al., 2008).
Clays
Clays are sheet-like phyllosilicate crystalline minerals with a layered structure of shared octahedral and tetrahedral sheets. Illite, kaolinite, and montmorillonite are three types of clays found in oil sands. Substitution of cations within the structure of these clays produces a variety of species of clays (Juma, 1998; Mercier et al., 2008)
Bulk deposits of clay are often present in the oil sands deposits. While bulk deposits of clay are largely avoided in the mining operations, clay is distributed in the ore and is therefore present in the hot water process. Fine clay particles interfere with the hot water process and the presence of fines in process water is undesirable. Particles of clays have negatively charged surfaces and sheet faces, and positive charges on the edge surfaces. Since the surface area of the sheet face is much larger than the surface area of the edge face, the negative charges dominate interactions between particles. Cations, including H3O+, may facilitate binding between sheets of clay. Clays may be non-swelling, for example kaolinite and illite clays, or swelling, for example montmorillonite clays.
Colloidal Suspension
When clays are introduced into the hot water process, they become defoliated and create the fines found in tailings. The charged surfaces of the fines form hydrogen bonds with water molecules. Fines in solution are a colloidal suspension. As the fines settle, they reach a point where steric forces impede further settling. When this point is reached, the suspension has the consistency of a gel and is called a “floc”. Colloidal suspensions may be described in terms of Gibbs free energy:ΔG=ΔH−TΔS  [Eq. 1]
In Eq. 1, ΔG is the change in Gibbs free energy, ΔH is the change in enthalpy, ΔS is the change in entropy, and T is the temperature, of the system. While systems seek a global minimum in G, a system may remain in a local minimum absent sufficient activation energy (“Ea”) to exceed the local minimum G and reach the lower global minimum. A floc is at a local minimum and will eventually settle out into clays which represent a global minimum free energy state. Colloidal suspensions are very stable and can last for years.
Bringing defoliated sheets of clay together again is analogous to adsorption of a molecule on a surface of another substance. At distance, the adsorbing molecule may not be attracted by the surface, and may actually be repelled. However, in close proximity, attractive Van der Waal forces may cause adsorption onto the surface. Similarly, the negatively-charged surfaces of clay sheets introduce steric repulsive forces when the fines particles are at distance, stabilizing the local minimum of the colloidal suspension. However, at close distances, the Van der Waal forces may become large enough to bring the sheets together. High ionic strength solutions tend to stabilize the settled clay and promote the settling of the floc state into the clay state.
The stability of the colloidal suspensions (and the ΔG value associated with its refoliation) is dependent on the ΔH of hydration of the clay sheets, the ΔH of refoliation into clays, and the ΔS of the system. Since the clays settle eventually, the ΔH of refoliation is sufficiently negative to overcome the negative ΔS of the transition from colloidal suspension to clay the ΔH of hydration. To move the equilibrium towards formation of ordered sheets (i.e. foliation), energy must be introduced to overcome the Ea of the transition. However, introduction of too much energy may move the colloidal suspension into an even higher free energy state, which is undesirable.
Fines are negatively charged and their surface area may be upwards of 100 m2/g, resulting in a high net negative charge of a suspension of fines. Following use in the hot water process, water includes OH−. The dissolved OH− contributes to charge interactions and interferes with the settling of colloidal suspensions into clays. Each clay sheet in the colloidal suspension interacts with water through hydrogen and Van der Waal bonds, dissipating surface charge energy. The energy of hydration of the colloidal suspension may, along with the steric forces, contribute to the activation energies to be overcome in refoliation.
Kaolinite may have a lower Ea to reform into clay than illite. Increasing the availability of cations to the clay formation may contribute to overcoming the Ea required to settle the colloidal suspensions. The availability of cations may be increased by acidifying the solution. At lower pH values, cations other than H3O+ are less likely to remain coordinated to OH− in solution and would be available for binding to negatively charged surfaces particles of fines. Further, the presence of H3O+ may decrease the amount of water surrounding fines, allowing particles of fines to settle closer together (a hydration layer does not need to be as thick to dissipate the negativity charge surfaces with the positive charge on the hydronium ions). This change in environment may reduce the Ea enough for the spontaneous settling of the colloidal suspension. If not, sufficient energy can be added to the system to overcome the remaining Ea, allowing settling to occur spontaneously into the lower free energy state. However, fines in general, and particularly fines that have formed a colloidal suspension, do not settle out of solution easily.